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Transcript of Petroleum System Bolivia
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8/11/2019 Petroleum System Bolivia
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Petroleum System
of
the Northern and Central
Bolivian Sub Andean Zone
P. Baby
O RS TO M
Grenoble, Fyance
I. Moretti
IFP
Ruei l Malmaison, France
B. Guillier
R. Limachi
E. Mendez
YPFB
Santa Cvu z, Bolivia
J. Oller
Petrolex
Santa Cruz, Bol ivia
O RS TO M
Quito, Ecuadol
M. Specht
Total
Paris-LaDefeme, France
Abstract
An
coupled study of the kinematics of thrusting and hydrocarbon maturation has been carr ied out in the
orthern and central sub-Andean belt of Bolivia to define the petroleum potential of the area. In
addition to the classic Devonian source rock (Tomachi-Tequeje formations to the north and Iquir i-Limoncito
formations in the central area), two other source rock intervals are recognized: the Retama Formation (Upper
Devonian-Lower Carboniferous) and the Copacabana Formation (Upper Carboniferous-Lower Permian).
These are the mo st prospec tive units in northern Bolivia and are of marine origin. The structural style varies
from north to south due to variations in the sedimentary column involved in the thrusts. The erogenic front
was guided by the northern boundary of a Paleozoic sedimentary wedge. In the Boomerang area, this
boundary is oriented obliquely to the regional shortening and controlled the developm ent of a prominent
transfer zone. To the north, the thrusts are wider and the amount of shortening increases. The western part of
the northern sub-Andean zone is character ized by a very thick Tertiary piggyback basin fi l l.
Two phases of hydrocarbon maturation are recognized. The fi rst began in Early Carboniferous and affected
mostly Devonian strata. Formation of structural traps during this period occurred rarely. The entire basin was
then deeply eroded in Permian-Jurassic t ime, causing any hydrocarbons that may have formed to be lost. The
second phase of maturation was contemporaneous with Andean deformation and with the resulting burial
under the Tertiary cover in the foreland basin and in piggybac k basins on thrust struc tures. The hydrocarbon
expelled during this period ma y fill the Andean anticlines. The know n source ro cks are not proven to be gas
prone, but current discoveries indicate a high gas to oi l ratio that may be due to secondary cracking in the
source rock. Because the ini tia l potential of the source rocks is low, expulsion of heavy compounds is expected
to be weak.
Resumen
n
U
estudio combinad o de la cinem atica de 10s corrimiento s y de la maduracion de 10s hidrocarburos ha
sido realizado en el sub-Andino norte y centre de Bolivia a fin de definir el potential petrohfero de la
u
zona. Ademas de las clasicas rotas madres devdnicas (formaciones Tomachi-Tequeje en el norte y formaciones
Iquiri -Limoncito en el centre), dos otras han sido definidas: la Formation Retama (Devonico sup. a
-
Carbonifero inf.) y la Form ation Copaca bana (Carbonifero superior a P&rnico inferior) que presentan el mejor
~
-
s
potential en el sub-Andino norte. Son de origen marino. El estilo estructural camb ia d e Norte a Sur, debido a
-
J:Tf
las variaciones de espeso r y de litologia en la column a sedimen taria implicada en 10s corrimien tos. El frente
------p-J
-
0-M
orogenico fue guiado por el borde norte de una cuiia sedimen taria paleozoica. En la zona de1 Boome rang, este
a 0-J
c-0
borde es ta orientado oblicuamen te en relation con la direction regional de acortam iento y control6 el desar-
EEzYzzzo
-0
rollo de una prominente zona de transferencia. Hacia el norte, 10s corr imientos son mas anchos y el acor-
g-
-r
m-0
-
6G
L-
-
-
Baby, P., I. Moretti, B. Guillier, R. Lima& i, E. Mendez, J. Oller, and M. Specht, 1995,
445
Petroleum system of the northern and central Bolivian sub-Andean zone, in A. J.
Tankard, R. Sukrez S., and H. J. Welsink, Petroleum basins of South A merica: AAPG
--.g~- .gfa p .& . -j-(-J&/
r
Memoir 62, p. 445458.
Fends DQc~ii 5
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446
Baby
et
al.
tamiento aumenta. La parte oriental del sub-Andino norte se caracteriza por un espeso relleno sedimentario
terciario contemporneo de la deformacin andina.
Dos fases de maduracin de hidrocarburos han sido reconocidas.La primera empieza
al
principio del
Carbonifero donde la columna sedimentaria paleozoica es espesa; afecta principalmente al Devnico. Las
trampas estructurales de este perodo son muy raras. Entre el Prmicoy el Jursico,la cuenca ha sido profunda-
mente erosionada, y los hidrocarburos generados probablemente han desaparecido. La segunda fase de madu-
racin es contempornea de la deformacin andina. Se debe a la fuerte subsidencia y sedimentacin terciaria en
la cuenca de ante-pas
y
en las cuencas transportadas por los corrimientos.
Los
hidrocarburos expulsados
durante este segundo perodo pueden haber rellenado anticlinales andinos.
No
se
ha
mostrado que las rocas
madres conocidas son propensas a generacin de gas, pero los hidrocarburos descubiertos indican una alta
proporcin gas-petroleo. Se interpreta como el resultado de un crcking secundario en
la
roca madre.
EI
bajo
potencial inicial de
las
rocas madres hace que la expulsin de
los
componentes pesados debe ser reducida.
INTRODUCTION
As in many compressional areas, the petroleum
potential of the sub-Andean zone of Bolivia depends on
the relative timingbetween structuring and maturation.
All the known source rocks are Paleozoic in age.
Therefore, a study of the petroleum system requires a
study of the geology, the kinematics of thrust emplace-
ment, and the kinetics of the various source rock matura-
tions. The results of these studies are presented. Differ-
ences in the sedimentary columns and structural styles
from north to south are emphasized, and the potential
source rocks are defined and their maturation docu-
mented. In some cases, we address the maximum
mount of
burial
before the Andean deformation; there is
considerable uncertainty because of the amount of
erosion.
REGIONAL SETTING
The sub-Andean zoneof Bolivia
is
a complex foreland
fold and thrust belt (Roeder, 1988; Sheffels, 1988, 1990;
Baby et al., 1989,1992,1993) that forms the eastern edge
of the central Andes mountains (Figure 1). t is bounded
along the edge of the Cordillera Oriental by the Main
Frontal thrust (CFP),whereas the orogenic front extends
below the Beni and Chaco plains to the east. Deformation
started in the late Oligocene and is continuing today
(Sempere et al., 1990). The material involved in sub-
Andean thrusting in Bolivia consists of a n Ordo-
vician-cretaceous series and an upper Oligocene-Recent
continental foredeep fill. The preorogenic sedimentary
series show lateral variations in facies and thickness that
play an important role in controlling the structural
geometry (Baby et al., in press). In its central part
(16 -17
S
lat), this fold and thrust belt forms a bend
(Santa Cruz bend) characterized by a prominent transfer
zone. From north to south, the structural geometry
displays variations in the amount and direction of short-
ening Figures 1,2).
Three structural zones are recognized 1) he northern
sub-Andean zone (13 -17 S lat), which is oriented
northwest-southeast; (2) the central sub-Andean zone
(17 -18 S lat), which changes from a northwest-
southeast to a north-south orientation; and (3) the
l o o
-15
2
Figure -Si m p l i f ied ectonic map of Bol iv ia showing the
location of the sub -Andean zone, a complex fold and thru st
belt, and the s tud ied cross sections. In its central part, the
Bo livian sub -Andean zone forms a bend (Santa Cruz bend)
characterized by the prom inent Boomerang-Chapare
transfer zone. In th e no rthern and cen tral part, the p ropag a-
tion of the orogenic front was g uided by the northern
bou ndary of th e Paleozoic sedimentary wedge.
SC,
Santa
Cruz; CFP, Main Frontal thrust; BCTZ, Bo om erang-Chapare
tran sfer zone.
southern sub-Andean zone (18 -22 S lat), which is
oriented north-south.This southern sub-Andean zone
is
not discussed
in
this paper.
Northern
Sub-Andean Zone
The Paleozoic succession (Figure
3) is
nearly complete
for the Ordovician-Permian and is unconformably
overlain by a Mesozoic sandstone cover up to 800 m
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Petroleunz
ystemof the Northem
and
Central Bolivian Sub Andean Zone
7
w
IAlto
Beni piggyback basin
NE
S Nl A FATIMA
O
km
-5
-
10
-15
A
sw
Late Paleogene-Neogene
Jurasic Cretaceous
Upper Garb.-Lower Perm.
Carboniferous
Devonian-Silurian-Ordovician
Cambrian Precambrian
NE
5
5
O O
km
-5
5
N E
w
Figure 2-Cross sections construc ted and balanced rom field studies and subsu rface data. The structural geom etry shows
important variations from north o south. The preorogenic sedimentary succession shows lateral variations of facies and
thickn ess that are impo rtant n contro lling he struc tural geom etry. C.F.P., main frontal thrus t. See Figur e
1
or locations.
thick. Toward the northeast, the thickness of the Ordovi-
cian decreases, the Silurian series disappears, and the
Devonian, Carboniferous, and Permian successions
progressively wedge out. At the top of the preorogenic
stratigraphic column, the continental foreland deposits
are up to
5000
m thick.
The thrusts are wide, with their wavelengtlw always
being more than 10 m Figure2A B). The western part
of the northem sub-Andean zone is characterized by a
very thick (6500-7000 m) Tertiary piggyback basin i l l
(Alto Beni syncline).The
main
dcollement level is at the
base of the Paleozoic column (Ordovician shales). The
other dcollement levels are shallower and are located in
Devonian, Carboniferous, and Permian shales. The
foredeep basement slopes at 4 .The maximum amount
of shorteningis 135km
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448
Babyetal.
S E R .
Figure SGe ne ral ized stratigraphic column for the
north ern sub-Andean zon e
of
Bolivia. Two m ain erosion al
unconformitiesare present n the Triassic (about 205Ma)
and upp er Oligocene (27 Ma) sections. The latter resulted
from the beginning of Andean deformation. The Middle
Triassic erosion prob ably occurred during an episode of
extension related to the init iationof Gondw ana fragmen ta-
tion. Two prom inent nondepositional unconformities are
present n the Cretaceous (144-68 Ma) and Paleocene
(53-27 Ma) sec tio ns .
Central Sub-Andean Zone
The preorogenic stratigraphic column is characterized
by a Paleozoic sedimentary wedge (Figure
4).
It
is
a
continuous succession, ranging from Ordovician to
Carboniferous in age and thinning northward onto the
Precambrian-Cambrian Brazilian shield. It is uncon-
formably overlain by 500 m of Mesozoic rocks and more
th n1600 m of upper OligoceneRecent deposits.
The orogenic front (Figures 1,2C) is characterized by
the Boomerang-Chapare transfer zone, which is inter-
preted as an oblique ramp. The propagation of the
orogenic front was guided by the northern boundary of
the Paleozoic sedimentary wedge, which is oriented
obliquely to the regional shortening diredion (Baby et al.,
in press). The major dcollement is located at the base of
the Paleozoic sedimentary wedge which slopes at 10 .
The maximum amount of shortening is
60
km
SOURCE ROCKS
Due to facies variations, a large number of formation
names have been introduced into the literature. Never-
theless, the stratigraphic column has been recently
rearranged and published by YPFB-ORSTOM (Sempere,
1990; Oller, 1992). We use this stratigraphicframework to
date the source rock formations.
To describe the various source rocks fully, the
Yacimientos Petroliferos Fiscales Bolivianos (YPFB)
database has been used and augmented with new data
collected by sampling outcrops and wells. The classic
source rock of the Bolivian sub-Andean zone is the
Devonian and probably the Silurian, but two other
Paleozoic formations are believed to have some
potential. Locally, an Upper Cretaceous source rock
(Flora Formation) with limited potential is known in the
northern sub-Andean zone.
Silurian
The Silurian source rock, of Wenlockian-Prodolian
age (430410 Ma), reflects siliciclastic deposition on a
marine platform. The Upper Silurian
is
known
in the
southern sub-Andean zone and foreland, in the central
part, and in a small part of the northern sub-Andean
zone, as shown in Figure 5A. Its thickness increases from
north to south, where it is up to 1500 m thick. The few
samples available to us do not permit a detailed evalua-
tion or even a confirmation of the original petroleum
potential of this interval. Nevertheless, marine type II
source rock characteristics are used for the modeling
procedure to quantify the maturity of a hypothetical
Silurian source rock.
Devonian
The Devonian succession is Lochkovian-Famenian in
age (410-360 Ma). Its depositional setting was essentially
the same as during the Late Silurian (marine platform
deposition). Its thickness increases toward the south,
where it is 1500 m thick south of the Boomerang. Its
regional distribution is shown in Figure 5B. The basin
extends northward into Peru and southward into
Argentina
and
Paraguay.
The
Devonian has been eroded
to the northeast; the age of erosion is discussed later.
Source rocks include the Tomachi and Tequeje forma-
tions in the north and the Iquiri and Limoncito forma-
tions in the central part of the basin.
We have not found immature Devonian samples and,
consequently, have not determined the kinetic parame
ters. Since the depositional environments of the Tomachi
and Iquiri formations are the same as the Retama
Formation of Carboniferous age, the type Retama section
described below
is
used to model the Upper Devonian
source rocks. The Tequeje and Limoncito formations
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1000m
Petroleum Systenzoft he Northern and Central Bolivian Sub-Andea n Zon e
N S
0 km CAMBRIAN
PRECAMBRIAN
449
Figure W ed im en ta r y wedge of the Boomerang area, in the central sub-Andean zone. The
two
main erosional unconform i-
ties (Triassic and upp er Oligocene) and the two prom inent nondepositional uncon formities Cretaceous and Paleocene) are
also pr esent in this region. See Figure 1 for location .
(Middle-Lower Devonian) correspond to more distal
deposits, therefore classic marine type II kerogen charac-
teristics have been chosen for the modeling.
Retama Formation
The shaly beds of the lower Retama Formation
of
Carboniferous age, also called Toregua Formation, show
reasonably good potential. Deposition started in the late
Famenian (360 Ma) and may have continued to the
Visean (327 Ma). The Toregua Formation, corresponding
to siliciclastic deposits of a marine platform, is an alterna-
tion of shaly and sandy beds which form the potential
reservoirs. This formation is present in the Madre de
Dios basin and in the northern sub-Andean belt (Figure
5C). It
is
about 500 m thick.
Retama source rock potential has been proven in the
Pando-X1 well (Madre de Dios basin). The total organic
carbon (TOC) content exceeds 2 wt. over a 100-m
interval, with a hydrogen index
(HI) of
more than 600
mg HC/g. All of the shaly beds have some potential
(TOC
>1
wt.
).
Kinetic parameters have been determjned on a sample
from the Pando-X1 well
(HI
= 660
mg
HC/g and T =
445C) using OPTIUN software (Espitali et al., 1985).
This T value is relatively high, and the maturity level
of this sample is not certain. Nevertheless, Rock-Eva1
pyrolysis
of
old source rocks, such as Devonian source
rocks
in
Algeria, commonly show high Tmaxvalues on
immature samples (C. Ducreux, 1993, personal commu-
nication). Optical data show a vitrinite reflectance value
of 0.5 ,
which confirms the immaturity
of
this sample.
On this basis, we use the computed kinetic parameters
for the modeling.
Copacabana Formation
The Copacabana Formation
of
Permian age is a
marine platform succession formed by an alternation of
shallow water (50-m) limestone, shale, and sandstone.
Deposition began in the Stephanian (307 Ma) and
continued to the Early Permian (270 Ma). It is present in
the Madre de Dios basin, in the northern sub-Andean
belt, and to a lesser extent in the central zone (Figure 5D).
Where it has not been eroded, it isup to 800 m thick.
Rock-Eval pyrolysis confirms the petroleum potential
of this formation. Recorded
TOC
values vary from
1
o 9
wt. , and the maximum HI is about 440 mg HC/g.
Unfortunately, we have insufficient data to define the
true thickness of the active source rock interval.
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450
Baby et
al.
A SILURIAN SOURCE ROCK
B DEVONIAN SOURCE ROCK
C TOREGUA SOURCEROCK
(Upper Devonian-Lower Carboniferous)
D
COPACABANA SOURCE ROCK
(Upper Carboniferous-Lower Permian)
MODELING
Becauseall the source rocks are older th nthe Andean
orogen, the key factor for exploration is the relative
timing
between maturation-migration and the f omtion of struc-
tures. Any early migration of hydrocarbons would
exclude the young structures as drillable prospects.
Kinematic and Geometric Hypotheses
The structures have been studied on the basis of three
balanced cross sections constructed from field studies
and subsurface data (Figure 2). Original data and discus-
sion of these sections are presented elsewhere (Baby et
al., 1989,1992,1993, in press). Nondepositional unconfor-
mities and erosion surfaces recognized
in
the field and
from seismic data have been dated as part of the YPFB
ORSTOM program (Sempere, 1990; Oller, 1992). The
two
main erosional unconformities are in the Triassic and the
upper Oligocene (Figures 3, 4). The latter
is
due to the
beginning of Andean deformation dated at 27 Ma
(Sempere et al., 1990).The age of the earlier erosion is not
well defined, but it is believed to have occurred during
Figure W eo g r ap h i c d is tr ib -
ution
of
source rocks in the
Bolivian sub -Andean basins.
(A) Silurian , (B) Devonian,
C) Toregua Formation
(Upper Devonian-Lower
Carbon iferous), and (D)
Copacabana Formation
(Upper Carboniferous-Lower
Permian). Shading ind icates
source rock distribution in
sub -Andean basin. SC, Santa
Cruz.
an episode of Middle Triassic extension that
is
known
in
the southern part of Bolivia and is related to the initiation
of Gondwana fragmentation (Oller and Sempere, 1990;
Soler and Sempere, 1993).A n age of 235 Ma is used in
our modeling.
Two prominent nondepositional unconformities
occur in the Cretaceous (144-68 Ma) and Paleocene
(53-27 Ma) stratigraphic sections (Figures 3,4).
Method
A
one-dimensional model (GENEX software) has
been used to determine maturity level
in
real and ficti-
tious wells. GENEX permits computation of the burial
history, the compaction based on porosity-depth rela-
tionships, the temperature history, and the maturation
and expulsion history.
Thrusting is not directly modeled by GENEX, so we
used a fast rate of sedimentation to simulate thrust
emplacement. This approach adequately describes the
burial and compaction history, but it also results in a
misleading thermal profile for a stratigraphic section
spanning a few million years. The temperature of the
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_ _ _
~
u
Petroleum System of the Northem and Central Bolivian Sub-Andean Zone
451
sw ANT.
LLlQUlMUNl
ANGOSTO NE
SNIA. PELADO QUIQUIBEY SNIA. CHINE SNIA FATIMA SNIA. EVA EVA
PRESENT
DAY
EVA EVA 1
Oil
EVA EVA 2
EVA EVA 3
Oil Gas CENOZOIC
B O Y A i
MESOZOIC BOYA 2
Gas
CARBONIFEROUS
DEVONIAN-ORDOVICIAN
PELADO 1
PERMIAN
CAMBRIAN-PRECAMBRIAN PELADO 2
FICTITIOUS WELLS QU QUIBEY
L Q M 1
I
LQM 2
PRE-ANDEANDEFORMATION
Figure 6-Oil and g as windows in the Ll iquimuni balanced cross section. (A) Present day and (B) pre-Andeamdeformation.
deposited sediment
is
assumed to be constant during the
sedimentation process, in contrast to the overthrust
sediments, which were already heated. There are two
ways to overcome
t s
problem. One can either impose a
steady-state solution or artificially increase the surface
temperature to simulate a temperature closer to that of
the overthrust sediments.
When a steady-state solution is imposed, the relax-
ation time is zero. The error resulting from active
thrusting is mainly a function of the thrust tluckness and
the rate of thrusting. The relationship of this error to the
rate of thrusting has been calculated (Endignoux and
Wolf, 19901, showing that the error is negligible for
normal rates of horizontal displacement. In this study,
the uncertainties about the timingof thrust emplacement
are so large (a few million years) that it would be inap-
propriate to use a more sophisticated approach.
Because thermal data are scare, an average heat flow
of 55 mW/m2 has been used for the modeling. No rifting
phase has been included. Neither do we have enough
data to describe heat flow variations through time. A
constant value for the basal heat flow is thus used. There
are also uncertainties regarding the amount of Triassic
erosion as well as the burial history. These constraints,
especially for Silurian and Early Devonian maturation,
limit us to a qualitative interpretation.
Maturation has been calculated using kinetic para-
meters derived from the Pando well (type Retama) for
the Toregua Formation and the Upper Devonian section
and default marine type II characteristics for the Lower-
Middle Devonian and Silurian. The maturation of this
kerogen is slower than a classic type
II
kerogen (e.g.,
Toarcian of the Paris basin in GENEX), resulting in an
estimated oil window
800
m deeper than the default
value for a gradient of about 25 C/km.
With GENEX software, the expelled quantities are
calculated as a function of the saturation in the source
rock. An expulsion threshold is defined by the user
(Forbes et al., 1991). When the saturation
is
less than
this
value, there is no expulsion. An arbitrary value of 15%
has been used
in this
study.
RESULTS
Northern Sub-Andean
Zone:
Lliquimuni
Cross Section
Geometry
The northem part of the sub-Andean zone is underex-
plored, with only four wells having been drilled. One of
these (LQM-X1) is included on the balanced cross section
(Figure 6) and another is located on the Boya trend, 50
km south of the section. The 235-Ma erosional event
appears to have stripped up to 800 m of section. Late
Oligocene erosion (27 Ma) was more limited; 100 m of
erosion is inferred for the missing section.
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o
1 y
s2:;
E
y
6
452 aby
et
al.
u
.-
HC
WINDOWS
1
i , , I
1 1
- .
I
i l i l I l I l
400 4 0
6 0
400 3 0 3d0
2 0
200
li0
100
6
COPACABANA
0.m.
Copacabana
ROCK-EVAL HYDRO GEN INDEX
6
B
Hydrogen Index S 2 m C mg HC/g initial TOC)
Figur e 7-Maturity in the Pelado structure, based on Rock-Eva1 pyro lysis and the su bsiden ce histor y of a fictitiou s well,
Pelado-2 (see Figu re 6 for location). (A) Subsidence history and hydrocarbon wind ow s. (B) Calculated and measured
hyd rog en ndex. (C) Calculated and m easured
Tmax
alues.
D)
Transform ation ratio of the various s ou rce rocks.
Subsidence and deposition of the deep Tertiary
piggyback basin of the Alto Beni syncline (Figure 2A)
was controlled by Andean deformation. Seismic data
show
two
unconformities within this basin
fill.
On the
eastern and western
flanks
of the Lliquimuni anticline,
the sedimentary cover is up to
6000
m
thick
(Figure 6).
The unconformities within the Miocene have not been
dated, but they are known to be related to the thrusting
phases of the Lliquimuni and Pelado structures. For
modeling purposes, we have used the ages of the
Tertiary formations and intra-Miocene unconformities of
the southern sub-Andean zone (Marshall and Sempere,
1991;
Gubbels et
al., 1993),
which we have correlated to
the northern sub-Andean area. Ages of 11and 6 Ma are
inferred for the Miocene unconformities (Figure
3).
The
geometry of the thrust structures shows a later phase of
deformation. We attribute ths out-of-sequence propaga-
tion phase to a 2.5-Ma event, which corresponds approxi-
mately to the age of deposition of the conglomeratic
Tutumo Formation. Farther east, deformation was
younger,
from
6 Ma to the present.
Petroleum System
The Silurian stratigraphy in our cross section is
known only west of the CFP (Figure 2A) and was not
modeled. The other three source rocks (Devonian
Tomachi and Tequeje formations, Toregua Formation,
and Copacabana Formation) are present to the west (see
Figure
5);
eastward the Permian and Carboniferous
sections are missing.
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Peetrolmm System of theNorthern aiid Centual
olivian
Sub-Andean Zone
453
Figure &Maturity in the foreland of the Lliqu imun i cros s section, based on the subs idence history of a fictitiou s well, Eva-
Eva
1
(see Figu re 6 for loc ation).
(A)
Subsidenc e history and h ydroc arbon windows. (B) Transfo rmation ratio of the various
source rocks.
Calibration was undertaken on the Pelado structure
for which geochemical data
(Tma
and HI) are available
(Figure 7). The maturation index, especially for the
Paleozoic source rocks, remains low along the western
anticlines.
On
the Pelado structure, Rock-Eval pyrolysis
has shown that the Copacabana Formation
is
immature
(Figures 7B, C), suggesting that it never was buried
under Tertiary sediments. Farther east, seismic data
record a Tertiary section that
is
uniform in thickness as
well as facies. These data suggest that the Pelado
structure developed as an early thrust that formed the
orogenic front and carried the Alto Beni piggyback basin.
Subsequently, the orogenic front migrated toward the
east where the Miocene cover o c m s in new structures
such as the Sierra Chine, Fatima, and Eva-Eva (Figure
6).
Figure 7A shows that in the Pelado structure, the
two
Devonian source rocks matured during the Permian.
Triassic erosion was not substantial and maturation
persisted to the present. The Copacabana and Toregua
formations are still immature. In the piggyback basin
westward of the structure, these two source rocks
matured during Andean deformation due to the thick
synorogenic deposits.
For comparison, Figure 8 shows the maturity in a ficti-
tiom well (Eva Eva-1) located
on
the foreland where only
the Lower and Middle Devonian source rocks are
present. The maturity appears to have occurred
in
the
Neogene and is attributed to foreland infilling. The oil
window was reached during Charqui deposition (6 Ma).
The transformation ratio of the Tequeje Formation is now
about
70
(Figure 8B).
The complete balanced cross section with the present
oil windows is shown in Figure
6.
There were two
phases of hydrocarbon generation. The first preceded
Andean deformation and began during the Carbonif-
erous and affected the thick Devonian sections. Only the
Devonian source rock matured during
this
period. The
second phase of hydrocarbon generation was initiated by
Andean
thrusting
during the late Oligocene and affected
all the source rocks. Burial maturation is attributed to
thick accumulations in the Alto Beni piggyback basin
and in the sub-Andean foreland basin durin g the
Neogene.
Up to 4500 m of Neogene sediments were deposited
prior to deformation in the eastern region. The rate of
sedimentation during the Neogene was very high. The
thermal transient effects resulted
in
deep oil windows
(about 5.4
k m
depth) and late maturation.
Northern Sub-AndeanZone:
Isiboro Cross Section
Geometry
The only petroleum play
in
this section
is
the Isiboro
frontal anticline. Other structures were deeply eroded
du e to the large amount of shortening
(58%)
hat
o c m e d in this area. Two wells have been drilled in the
Isiboro anticline, and one (SSA-X1) is included in the
balanced cross section (Figure9). Compared to the major
Triassic erosion, the late Oligocene unconformity was
relatively insigruficant. The amount of Triassic erosion
increased from southwest to northeast, where up to 700
m of section were removed.
Petroleum System
The Copacabana, Retama, and Devonian Tomachi
and Tequeje source rocks are believed to have the same
characteristics as their northern counterparts. The
Silurian with marine affinities
is
included as a potential
source rock interval (see Figure 5). Eastward, the
Permian and Carboniferous intervals are missing.
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10/14
454 Baby et
al.
1s 2
IS. 3
I
1
FICTITIOUSWELLS
Figure M i l nd g as window i n the lsobo ro balanced cross section . (A) Present day and (B) predndean deformation.
Figure
10
shows a comparison between the maturity
along the edge of the external part of the preorogenic
basin (well IS.
1)
and along its internal part where the
Devonian section
is
thick (wellIS.3).
Two phases of hydrocarbon generation occurred.
Figure 9 shows the source rocks that were matured
before and during Andean deformation. Maturation of
the Silurian and Devonian sections started during the
Devonian
in
the internal part where the Paleozoicis very
thick (up to
4000
m). Toward the east, burial resulted
from foreland deposition that was contemporaneous
with Andean deformation. As in the Lliquimuni cross
section, the oil window is relatively deep (up to 5 km
due to the large amount of sedimentation.
Central Sub-Andean Zone: Boomerang
Cross Section
Geometry
This cross section spans the principal hydrocarbon
province of Bolivia. It is well constrained by subsurface
seismic data and by the Santa Rosa X and San Juan
X
wells (Figure
11).
The Boomerang zone is formed by the
deformed border of the Paleozoic sedimentary wedge.
Well data permit quantification of late Oligocene erosion;
the Upper Cretaceous section was thinned to about
300
m. Triassic erosion was substantial and removed at least
2km
of Paleozoic deposits.
Petroleum System
The Copacabana and Toregua formations are
missing
(Figure
5).
We used two source rocks in our model: the
Devonian (Limoncito and Iquiri formations) and the
Silurian. Rock-Eva1 data from the Santa Rosa
X2
well
record
a Tm
of about 440C in the Limoncito Formation,
which corresponds to the beginning of the
oil
window for
this
typeof organic matter (Retama kinetic parameters).
The subsidenceand
maturity
measured and calculated on
the San Juan
X
well are shown %Figure
12.
Figure
11
sullunarizes these maturation results on the
balanced cross section, showing present-day and pre-
Andean
oil
windows. Even before Andean deformation,
the Silurian and Lower-Middle Devonian source rocks
were mature. Maturation started at the beginning of the
Carboniferous and expulsion was completed by the end
of the Carboniferous.No structural traps are known for
this period, thus limiting exploration opportunities to
stratigraphic plays. These source rocks are now in the gas
window. During the Andean deformation, only the
Upper Devonian section entered the oilwindow.
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4r
;
K
Petroleum
ystemof
th Northem
and Central olivian Sub-Andean Zone
HC Windows
. . . . . . . . . . . . . . . . .
sk-*i
il Window
l o ? ~
I I l I I I I I 1 I I
l
Il
450 400 350 300 250 200 150 100 50 O
Time
(Ma)
A
HCWindows
System
Series
O
Oil Gas Zone
10
450 400 350 300 250 200 150 100 50
Time
(Ma)
B
Formations
Formations
Figure 10-Maturity in the lsoboro cross section, comp aring
(A)
fictitious w ell IS.
1
in the foreland with a th in Paleozoic
section to (B) f icti tious well
IS. 3
in the hinterland with a thick Paleozoic section. See Figure 9 for location s of wells.
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12/14
456
BOOM. 1
BOOM. 2
BOOM.
3
Baby
et al.
FICTITIOUS
WELLS
A
BOOM.3 BOOM.2 Bo oM .1 NE
SAN
JuAN-x2
SANTA ROSA-=
sw
PRESENT,DAY
CENOZOIC
MESOZOIC
CAMBRIAN-PRECAMBRIAN
DEVONIAN-SILURIAN-ORDOVICIAN
Oil window
Oil Gas zone
CARBONIFEROUS
i
Gaswindow
I
PRE-ANDEAN
DEFORMATION
r5
I
I 15
10 k m
-
Figur e 11-Oil and gas windows in the Boo merang balanced cros s section. (A) Present day and
B)
pre-Andean deformation.
FINAL
GASTO
OIL RATIO
The present proven reserves show that the sub-
Andean zone, and especially the Boomerang area, are
mainly gas prone. AU the major fields (Carasco, Katari,
Palometa, Santa Rosa, Sirari, Vivora, and Yapacani)
produce gas and condensate. Nevertheless, the source
rocks are of marine origin and not
gas
prone. We
attribute this to the very low initial potential of the source
rock. Any
oil
generated has not yet been expelled but
rather remains in the source rock where burial and matu-
ration continue. Lighter compounds are formed and
migrate into the Andean structures.
CONCLUSIONS
In the northern and central sub-Andean zone of
Bolivia, the propagation of the orogenic front was guided
by the northern boundary of the Paleozoic sedimentary
wedge. In the Boomerang area, this boundary is oriented
obliquely to the regional shortening and it controlled the
development of a prominent transfer zone. To the north,
the thrusts are wider and the amount of shortening
increases. The western part of the northem sub-Andean
zone is characterized by a thick Tertiary piggyback basin
fill.
Geochemical analysis documents four source rocks in
the Paleozoic sedimentary wedge: the Retama Formation
(Upper Devonian-Lower Carboniferous), the Copaca-
bana Formation (Upper Carboniferous-Lower Permian),
the Devonian deposits (410-360 Ma), and probably the
Silurian section (430410 Ma). Specific kinetic parameters
for the Retama Formation show a later maturation
compared to other marine type II source rocks.
Ourmodeling emphasizestwophases of hydrocarbon
generation. The first occurred from the Devonian to
Carboniferous due to deepening of the Paleozoic basin.
Hydrocarbons expelled before the Triassic are believed
to have filled stratigraphic traps. Because of the absence
of structures and widespread erosion, most of these
hydrocarbons have probably been lost. The second phase
of maturation is attributed to burial by Tertiary deposi-
tion in the foreland and piggyback basins.
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~~ ~~~~ ~
Petroleum
ystem
of the Northern and Central olivian Sub-Andean Zone
457
o
o
HCWindows
System
Series
0 -
System
Series
O
. .. . .
b
450 400 350
300 250
200
150 100 50
O
HC WINDOWS
;2 Oil Window
Oil Ga s Zone
Gas Window
*y
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458
Baby et
al.
Acknowledgments This study resulted f rom
a
research
convention between
YPFB,
ORST OM, and G P . We thank C.
Dzicreux B E K I P )
for
the Rock-Eva1 analyses and kinetic
parameters and
J. .
Pittion Total) or the geochemical reszilts
interpretation. G E N E X is
an IFP
commercial product
marketed
by
BEICIP. We thank
Jim
Maloney, Mike Perkins,
and
mi
anonymous reviewer
or
their comments on
an
earlim
version
of
this manuscript.
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Authors
Mailing
Addresses
P.
Baby
ORSTOM
Laboratoire de Godynamique des Chanes Alpines
15,rue Maurice Gignoux
38031 Grenoble
France
I.Moretti
IFP,
BP
311
92305 Rueil Malmaison
France
B.
Guillier
ORSTOM
AP. 171106596
Quito
Ecuador
R.
Limachi
E. Mendez
WFB
C.P. 1659
Santa Cmz
Bolivia
J.
Oller
Petrolex
C.P.
3969
Santa
Cruz
Bolivia
M.
Specht
Total
Cedex
47
92509 Paris-La Defense
France